tune diagonalization-tweak so it's fast on SSE2 too
tune diagonalization-tweak so it's fast on SSE2 too

Without this change the pivot-around-b tweak would cause a performance
regression on pre-AVX2 Intel archs. With this adjustment, the new
diagonalization is faster across the board.

cryptocorrosion/cryptocorrosion,cryptocorrosion/cryptocorrosion